Additive manufacturing techniques and processes generally involve the buildup of one or more materials, e.g., layering, to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including, e.g., freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques may be used to fabricate simple or complex components from a wide variety of materials. For example, a freestanding object may be fabricated from a computer-aided design (CAD) model.
A particular type of additive manufacturing is commonly known as 3D printing. One such process, commonly referred to as Fused Deposition Modeling (FDM), or Fused Layer Modeling (FLM), comprises melting a thin layer of thermoplastic material and applying this material in layers to produce a final part. This is commonly accomplished by passing a continuous, thin filament of thermoplastic material through a heated nozzle, or by passing thermoplastic material into an extruder, with an attached nozzle, which melts the thermoplastic material and applies it to the structure being printed, building up the structure. The heated material may be applied to the existing structure in layers, melting and fusing with the existing material to produce a solid finished part.
The filament used in the aforementioned process may be produced, for example, by using a plastic extruder. This plastic extruder include a steel screw configured to rotate inside of a heated steel barrel. Thermoplastic material in the form of small pellets may be introduced into one end of the rotating screw. Friction from the rotating screw, combined with heat from the barrel, may soften the plastic, which may then be forced under pressure through a small round opening in a die that is attached to the front of the extruder barrel. In doing so, a “string” of material may be extruded, after which the extruded “string” of material may be cooled and coiled up for use in a 3D printer or other additive manufacturing system.
Melting a thin filament of material in order to 3D print an item may be a slow process, which may be suitable for producing relatively small items or a limited number of items. The melted filament approach to 3D printing may be too slow to manufacture large items. However, the fundamental process of 3D printing using molten thermoplastic materials may offer advantages for the manufacture of larger parts or a larger number of items.
A common method of additive manufacturing, or 3D printing, may include forming and extruding a bead of flowable material (e.g., molten thermoplastic), applying the bead of material in a strata of layers to form a facsimile of an article, and machining the facsimile to produce an end product. Such a process may be achieved using an extruder mounted on a computer numeric controlled (CNC) machine with controlled motion along at least the x-, y-, and z-axes. In some cases, the flowable material, such as, e.g., molten thermoplastic material, may be infused with a reinforcing material (e.g., strands of fiber or combination of materials) to enhance the material's strength.
In some instances, the process of 3D printing a part may involve a two-step process. For example, the process may utilize a large print bead to achieve an accurate final size and shape. This two-step process, commonly referred to as near-net-shape, may begin by printing a part to a size slightly larger than needed, then machining, milling, or routing the part to the final size and shape. The additional time required to trim the part to a final size may be compensated for by the faster printing process.
Thermoplastic materials used in additive manufacturing processes may generally expand when heated and contract or otherwise shrink when cooled. The amount the material expands and contracts per unit of distance per unit of temperature is generally referred to as the Coefficient of Thermal Expansion (CTE). When a material is heated above its melting point, the material typically will soften and subsequently re-harden or cure when again cooled. This transition from a melted material to a solid generally occurs at a relatively high temperature. The additive manufacturing processes discussed herein generally occur at or near this melting point. Once a printed part begins to cool and harden, the part may shrink or otherwise contract as the part's temperature continues to drop until the part reaches the ambient temperature of the surrounding environment. Since in the near net shape process, the printed part will generally be machined at ambient temperature and since the cooling and shrinking process may cause a significant reduction in the size of the printed structure, especially for large parts, in many cases it is necessary to print the part to a relatively larger size to ensure that the part size after cooling maintains a sufficiently large dimension to maintain trim stock to support the machining or trimming process required to achieve the final net size.
Fiber filler such as glass or carbon fiber may be commonly used in thermoplastic materials for applications such as industrial tooling. Fiber reinforcement in thermoplastic materials may introduce additional complexity. During the extrusion and printing process, fibers within the softened material tend to align with the direction of the print bead. This fiber alignment tends to reduce the expansion and contraction along the direction of the print bead as compared to expansion and contraction in directions perpendicular to the print bead. Thus the printed part, which may include print beads oriented in a multitude of directions, will normally expand and contract as a reaction to temperature changes at different rates in different directions.
Such asymmetric expansion and contraction may affect both the initial printing process, as the part transitions from a generally liquid state to a generally solid state at room temperature, as well as when a room temperature part is machined to its final net size and shape, which may be heated for use at an elevated temperature.
Industrial tooling normally needs to function at a pre-determined size and shape and in many cases this size and shape must be correct at an elevated working temperature. Therefore, a method must be employed to adjust the printing and trimming processes to allow for the normal expansion and contraction that occurs with thermoplastic materials and specifically with the asymmetric expansion and contraction that occurs with a fiber reinforced thermoplastic material(s).
In the practice of the aforementioned process, a major deficiency has been noted. The one way of addressing these requirements is to modify the CNC print and CNC trim programs to allow for shrink in the print process and expansion in the trim process, creating new modified programs which are then processed. This can be a difficult and time consuming programming process particularly when dealing with fiber reinforced thermoplastics, which may require that, among other things the part be modified at different rates in different directions. Especially since 3D printing software today does not generally support these functions. Also, the ambient temperature is a parameter that must be used in developing the modified programs and if the actual temperature when the process is conducted differs from that used in developing the modified programs, errors can occur. Another difficulty may be with dealing with a multitude of CNC programs for the same part that differ only by small amounts. Such variations can be confusing to operators and lead to errors.